Friday, August 21, 2015

3 Phase Induction Motors Objective Questions with Answers: Part-2

[1] Which of the following statements about a three-phase squirrel-cage induction motor are false?
(a) It has no external electrical connections to its rotor
(b) A three-phase supply is connected to its stator
(c) A magnetic flux which alternates is produced
(d) It is cheap, robust and requires little or no skilled maintenance

[2] Which of the following statements about a three-phase induction motor are false?
(a) The speed of rotation of the magnetic field is called the synchronous speed
(b) A three-phase supply connected to the rotor produces a rotating magnetic field
(c) The rotating magnetic field has a constant speed and constant magnitude
(d) It is essentially a constant speed type machine

(1) C only(2) B only(3) A only(4) All of the above(5) A and C only

Ans: 2

[3] Which of the following statements is false when referring to a three-phase induction motor?
(a) The synchronous speed is half the supply frequency when it has four poles
(b) In a 2-pole machine, the synchronous speed is equal to the supply frequency
(c) If the number of poles is increased, the synchronous speed is reduced
(d) The synchronous speed is inversely proportional to the number of poles

(1) All of the above(2) D only(3) B only(4) None of the above(5) A and C only

[5] In a three-phase induction motor. Which of the following statements are false?
(a) The slip speed is the synchronous speed minus the rotor speed
(b) As the rotor is loaded, the slip decreases
(c) The frequency of induced rotor e.m.f.’s increases with load on the rotor
(d) The torque on the rotor is due to the interaction of magnetic fields

(1) All of the above(2) C only(3) B only(4) A and C only(5) B and D only

Ans: 3

[6] In a three-phase induction motor. Which of the following statements are false?
(a) If the rotor is running at synchronous speed, there is no torque on the rotor
(b) If the number of poles on the stator is doubled, the synchronous speed is halved
(c) At no-load, the rotor speed is very nearly equal to the synchronous speed
(d) The direction of rotation of the rotor is opposite to the direction of rotation of the magnetic field to give maximum current induced in the rotor bars

[9] The starting torque of a simple squirrel-cage motor is:
(a) Low
(b) Increases as rotor current rises
(c) Decreases as rotor current rises
(d) High

Ans: A

[10] The slip speed of an induction motor:
(a) is zero until the rotor moves and then rises slightly
(b) is 100 per cent until the rotor moves and then decreases slightly
(c) is 100 per cent until the rotor moves and then falls to a low value
(d) is zero until the rotor moves and then rises to 100 per cent

Dear sir,as per my knowledge synchronous speed is inversely proportional to number of polesNs = 120f/pBut you have given in the answer it is false statement for question -3, so plz clarify the same.Thank you